1. Field of the Invention
The invention provides a variety of optical devices including digital and linear amplitude optical modulators, optical shutters, tunable optical filters, and optical cutoff limiters.
2. Description of the Prior Art
In a classic form of electro-optic modulator an electro-optic medium is disposed between a pair of crossed polarizers along a light beam path and is oriented so that an electric field is applied to the medium at an angle of 45.degree. with respect to the polarizer pass directions. Linearly polarized light emerging from the first polarizer divides into equal components along the two principal directions of the electro-optic medium. The electro-optic effect produced by the applied electric field causes these two components to travel through the medium at different velocities and hence gradually become out of phase. An elliptically polarized light beam emerges from the medium, and the second polarizer passes only the light component parallel to its transmission direction. By varying the voltage applied to the electro-optic medium, the polarization of the beam can be made more or less elliptic, and as a result the amplitude of the output beam can be modulated accordingly. Further details concerning this type of modulator may be found in a paper by Donald F. Nelson, "The Modulation of Laser Light," Readings from Scientific American, W. H. Freeman and Company, June 1968, pages 332-338.
More recently, laser beam modulators have been developed utilizing the Stark effect (the change in the spectrum of a molecule when subjected to an electric field). In such a modulator, a laser beam is passed through a cell containing gas molecules having an absorption resonance near the frequency of the laser beam, and a dc electric field is applied to the cell to shift, or tune, the absorption resonance so as to overlap the frequency of the laser beam. A modulating signal applied to the cell varies the absorption resonance about its steady state value, producing modulation of the laser beam. A particular Stark-tuned modulator for modulating an isotopic carbon dioxide laser beam and utilizing ammonia gas as the Stark medium is disclosed in U.S. Pat. No. 4,085,387, issued Apr. 18, 1978 to Charles K. Asawa et al and assigned to the assignee of the present invention.
In the past electronically tunable optical filters have been provided wherein a spatially periodic dc electric field applied to a birefringent medium disposed between appropriately oriented polarizers is utilized to electronically program the filter with a wide variety of transmission characteristics. For further details concerning this type of electronically tunable filter, reference may be made to patent application Ser. No. 864,356, filed Dec. 27, 1977, by Douglas A. Pinnow et al., now U.S. Pat. No. 4,197,008, and assigned to the assignee of the present invention.
An optical isolator utilizing the Zeeman effect has been devised to protect a laser system from undue absorption of scattered light energy. Such an isolator employs a Faraday rotator disposed along a laser beam path between a pair of polarizers having their respective pass directions for polarized light oriented at 45.degree. with respect to one another. Application of an external magnetic field to the Faraday rotator causes an absorption line in the rotator material to undergo Zeeman splitting into two components which are shifted in opposite directions away from a central frequency. The strength of the magnetic field is adjusted, depending on the type of rotator material, so that light at the central frequency between the split absorption components passes through the rotator. Further details concerning this optical isolator may be found in U.S. Pat. No. 4,033,670, issued July 5, 1977 to George A. Tanton et al.